A typical extracorporeal surgical blood circuit such as the blood circuit associated with a heart-lung machine in cardiovascular bypass surgery contains, among other elements, a cardiotomy reservoir and a softshell reservoir. The cardiotomy reservoir receives blood and debris sucked from the surgery field, and filters and defoams it for oxygenation in the heart-lung machine and eventual return to the patient's circulatory system. The filtered and defoamed cardiotomy blood is mixed in the softshell reservoir with venous blood from the patient's vena cava, and the mixed blood is then conveyed to the blood pump of the heart-lung machine.
Because the total blood supply to the softshell reservoir is sometimes more and sometimes less than the blood demand of the heart-lung machine's blood pump, the softshell reservoir must store enough blood to absorb any blood input fluctuations while supplying a steady blood flow to the pump.
A major problem in cardiovascular bypass surgery is the priming of the blood circuit. Before pumping blood through the heart-lung machine's oxygenator and back into the patient's circulatory system, all air must be purged from the blood circuit. This is done by priming, i.e. filling the blood circuit with saline solution. When the patient is connected to the heart-lung machine, the saline solution in the circuit is discharged into the patient's circulatory system and eventually mixes with the patient's blood. Because this at least temporarily makes the blood in the patient's circulatory system nonhomologous, and even after thorough mixing leaves the patient anemic for several days, it is important to minimize the priming volume of the blood circuit in every possible way.
One of the components that contributes significantly to the priming volume is the tubing that interconnects the various operational elements of the blood circuit. For that reason, and also for convenience of set-up and disposal as well as for economic considerations, it is desirable to consolidate as many blood circuit elements as possible into a single element that requires no interconnecting tubing and has a low priming volume.
Another problem in the blood circuit is that cardiotomy blood that remains too long in proximity to the cardiotomy filter-defoamer elements tends to start clotting and causing difficulties in the circuit.